The present invention relates generally to a vehicular power train including an automatic transmission and an engine which, with the same throttle position, can produce different output torques in response to a change in another operating variable other than the throttle position. More particularly, the present invention relates to a system for controlling the a servo activating hydraulic pressure acting on a servo motor of a shifting friction unit, such as a clutch or a brake, after a shift command signal has been generated by an automatic transmission control unit.
It is the common practice in automatic transmission design to provide a regulator valve which generates a hydraulic fluid pressure (i.e., a line hydraulic fluid pressure) variable in a pattern predetermined in response to throttle position. The known automatic transmission also includes a control unit or module which generates a shift command when predetermined variables, such as vehicle speed and throttle position, attain a predetermined relationship. Upon receiving the shift command, one of shift valves shifts to initiate supply of the line pressure toward an associated servo motor for a friction unit, such as a clutch, assigned for shifting. Thus, a hydraulic fluid pressure acting in the servo motor of the friction unit, which may be called a servo activating hydraulic fluid pressure, starts to rise. In order to moderate such a rise in the servo activating hydraulic fluid pressure, there is provided an accumulator in association with a hydraulic fluid line which leads to the servo motor from the associated shift valve.
With the same throttle position, the engine output torque reduces in response to a change in engine ignition timing to a retard ignition position which occurs upon detecting a tendency of an engine to knock or a change in compression ratio to a relatively low compression value which occurs upon detecting a change in operating condition or a change in A/F ratio to the lean air fuel mixture side. In the case of an internal combustion engine provided with a supercharger, with the same throttle position, the engine output torque increases in response to a change from inoperative state of the supercharger to operative state thereof.
If the same servo activating hydraulic fluid pressure is used to activate a friction unit which is asigned for shifting after the engine output torque has reduced, the period of time during which engagement of the friction unit proceeds becomes short, causing considerable shock to occur. If the same servo activating hydraulic fluid pressure is used after the engine output torque has increased, the engagement capacity of a friction unit runs short.
An object of the present invention is to provide a system for controlling servo activating hydraulic fluid pressure applied to a shifting friction in such a manner as to effect a shockless shift in an automatic transmission even if there occurs a change in engine output torque of the above-mentioned character.